1. Technical Field
The present invention relates to a cathode ray tube, and more particularly to a circuit for controlling a cathode heater of a cathode ray tube by rapidly warming the cathode heater when an electronic appliance using a cathode ray tube is turned on or when such an appliance returns to a power-on mode from an operation mode which has cut off the electric power supply to a Display Power Management System (DPMS) cathode heater.
2. Related Art
A cathode ray tube is widely used as a video display device in electronic appliances such as monitors and television sets. An electric power supply energizes the cathode heater of an electron gun provided in the neck portion of the cathode ray tube. The energized cathode heater causes the cathode to be heated and to generate thermoelectrons. The thermoelectrons generated from the cathode are controlled, converged and accelerated by a plurality of grids provided in the electron gun that they strike the fluorescent surface in front of the electron gun, and thereby display an image.
Before the cathode is heated up to a normal operating temperature, thermoelectron emission is unstable and the cathode ray tube cannot present a stable image. Conversely, after the cathode is heated up to a normal operating temperature, the cathode ray tube can present a stable image since thermoelectron emission becomes stable.
A slow cathode warm-up time has been considered undesirable by the public because this causes a delay between the moment the cathode ray tube is turned on and the moment an acceptable picture appears on the face of the cathode ray tube. Several techniques have been attempted to decrease the time required to heat a cathode in a cathode ray tube, and to thereby decrease the time required to wait for an acceptable picture.
The Display Power Management System (DPMS) was proposed by the Video Electronic Standard Association (VESA) of the United States of America. DPMS is an electric power supply management system for reducing electric power consumption of a video display system such as a monitor, which is a computer peripheral, in accordance with a use state of a computer system.
According to DPMS, electric power management is performed in four modes by selectively outputting and cutting off a horizontal synchronization signal and a vertical synchronization signal in the main body of the computer system according to a use state, and by corresponding with the horizontal synchronization signal and vertical synchronization signal from the main body of the computer system in the video display device. The four modes are classified as a power-on state mode, a stand-by state mode, a suspension state mode and a power-off state mode.
The video display device is operated in the power-on state mode when both the horizontal and vertical synchronization signals are input from the main body of the computer system. The video display device is operated in the stand-by state mode when only the vertical synchronization signal is input. The video display device is operated in the suspension state mode when only the horizontal synchronization signal is input. The video display device is operated in the power-off state mode when neither the horizontal nor the vertical synchronization signals are input. In the power-off state mode, electric power consumption must be less that 5 W.
One technique used to cause the cathode to reach an operating temperature quickly requires that the cathode be kept warm all the time, by means of a bleeder current. This technique has been called an "instant-on" feature and has been provided on some televisions by television receiver manufacturers. With this feature, a viewable picture is obtained on the cathode ray tube almost instantaneously with the turn-on of the television. The bleeder current used to accomplish this feature constantly maintains the cathode heater at a near-normal operating temperature. Thus, in effect, the cathode ray tube is never completely turned off. One example of this "instant-on" feature is disclosed in U.S. Pat. No. 3,767,967 for Instant-On Circuitry for AC/DC Television Receivers issued to Luz. This feature might be considered to be wasteful of electrical energy since the television is constantly drawing electrical power. Also, this feature could present a fire hazard.
Another technique used to cause the cathode to reach an operating temperature quickly requires a modification in the design of the cathode heater. Some examples of this are disclosed in U.S. Pat. No. 3,881,126 for Fast Warm-Up Cathode Assembly issued to Boots et al., U.S. Pat. No. 5,424,620 for Display Apparatus for Displaying Pictures Virtually Instantaneously issued to Cheon et al., U.S. Pat. No. 3,883,767 for Heater for Fast Warmup Cathode issued to Buescher et al. and U.S. Pat. No. 4,379,980 for Quick Operating Cathode issued to Takanashi et al. Also, the cathode design may be modified in order to enable the cathode to reach an operating temperature quickly. Some cathode design modifications are disclosed in U.S. Pat. No. 3,947,715 for Fast Warm Up Cathode for a Cathode Ray Tube issued to Puhak, U.S. Pat. No. 4,675,573 for Method and Apparatus for Quickly Heating a Vacuum Tube Cathode issued to Miram et al. and U.S. Pat. No. 4,388,551 for Quick-Heating Cathode Structure issued to Ray.
Another technique used to cause the cathode to reach an operating temperature quickly is to use a fast-acting heater circuit. This type of circuit is typically designed to energize the cathode heater in such a way as to cause the cathode heater to become warm quickly. The object of this type of circuit is to cause the cathode heater to become warm as quickly as possible, thereby decreasing the overall time required to cause the cathode to reach a normal operating temperature.
One example of a fast-acting heater circuit is disclosed in U.S. Pat. No. 3,886,401 for Apparatus for Accelerating Cathode Heating issued to Berg. This circuit includes at least two positive temperature coefficient (PTC) resistors. Upon activation of this circuit, the room-ambient resistance of the element allows passage of a surge of current which then decreases to the normal cathode heater operating level as the temperature of the resistive element increases. Because PTC resistors cause heat-related energy consumption, this circuit might be considered to be a poor utilizer of electrical energy. Also, this circuit might be considered to lack precision, due to the PTC resistors.
A second example of a fast-acting heater circuit is disclosed in U.S. Pat. No. 3,982,153 for Rapid Warm-Up Heater Circuit issued to Burdick et al. This circuit utilizes a degaussing circuit, a temperature responsive resistive element (such as a PTC resistor) and at least two transformers. Some might have the opinion that this circuit lacks precision and that this circuit poorly utilizes electrical energy, due to the use of the temperature responsive resistive element. Others might believe that this circuit is undesirable due to the high cost associated with the use of two transformers. Still others may dislike the inherent limitation presented by the fact that a degaussing circuit is required.
Cathode and cathode heaters are used in fluorescent light tubes. Some of the developments in the rapid warn-up of fluorescent light tubes are disclosed in U.S. Pat. No. 4,857,808 for Modified Impedance Rapid Start Fluorescent Lamp System issued to Lally et al., U.S. Pat. No. 5,5,010,274 for Starter Circuits for Discharge Lamps issued to Phillips et al., U.S. Pat. No. 5,583,395 for Fluorescent Device Having a Fluorescent Starter Which Precisely Controls Heating Time and Absolute Synchronisn of Fire Point issued to Lu, U.S. Pat. No. 5,440,205 for Fluorescent Lamp Starter Having a Transistor Base Control Means issued to Tahara et al., U.S. Pat. No. 5,319,281 for Fluorescent Tube Heating and Starting Circuit issued to Roth, U.S. Pat. No. 4,661,745 for Rapid-Start Fluorescent Lamp Power Reducer issued to Citino et al. and U.S. Pat. No. 3,731,142 for High-Frequency Fluorescent Tube Lighting Circuit with Isolating Transformer issued to Spira et al.